A traditional form of US folksong, expressive of the unhappiness of blacks in the Southern states. The usual subject matter is love, the troubles that beset the singer or a nostalgic longing for home, coupled with an acceptance of life as it is. Blues usually consist of 12 bars made up of three four-bar phrases in 4/4 time. They can be vocal or instrumental and have had significant influence on Jazz.

from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

African-American music that originated in the work songs and Negro spirituals of the rural American South in the late 19th century. It is usually of a slow to moderate speed and characteristic features include a 12-bar (sometimes 8-bar or 16-bar) construction and a syncopated melody line that often includes ‘blue notes’ (quarter tones lying between the minor and major third of the scale – as found on some African five-note xylophones – or between the minor and major seventh). The lyrics are melancholy and tell tales of woe or unhappy love. The guitar is the main instrument, although the harmonica and piano are also common. Blues guitar and vocal styles have played a vital part in the development of jazz, rock, and pop music in general.

1920s–1930s The rural or delta blues was usually performed solo with guitar or harmonica, by such artists as Robert Johnson and Bukka White, but the earliest recorded style, classic blues, by such musicians as W C Handy and Bessie Smith, was sung with a small band.

1940s–1950s The urban blues, using electric amplification, emerged in the northern cities, chiefly Chicago. As exemplified by Howlin' Wolf, Muddy Waters, and John Lee Hooker, urban blues became rhythm and blues.

1960s The jazz-influenced guitar style of B B King inspired many musicians of the British blues boom, including Eric Clapton.

1980s The ‘blues noir’ of Robert Cray contrasted with the rock-driven blues playing of Stevie Ray Vaughan.

In classical music, composers such as Maurice Ravel, Aaron Copland, and Michael Tippett have used aspects of the blues (especially the ‘blue note’) in various of their works, the Ravel piano trio in A minor (1914) being a notable example.